The present Application relates to a process for producing vacuole-free polymer granules wherein, after having been extruded and before being cut, the polymer is cooled at xe2x89xa650xc2x0 C./sec to just above the glass transition temperature.
The extrusion of the polymer into a specific, in most cases cylindrical, shape is one of the final processing steps in the production of polymers. After the extrusion, the product is then cooled, cut and, if necessary, cooled once more before it is in the form of a saleable product.
One of the requirements placed on this end product is that it should contain no vacuoles, i.e. small gas bubbles, because these have an adverse effect during subsequent processing. Moreover, transparent polymer granules containing vacuoles are visually less appealing.
WO 96/26241 discloses a process for producing polymer granules wherein the strand of polymer, after having been extruded in a water bath at a temperature of between 44xc2x0 C. and 95xc2x0 C., is cooled and then cut. The resulting granules contain less than 10 vol. % of vacuoles. However, this value is still not satisfactory.
A process has now been found whereby vacuole-free granules, i.e. granules having a vacuole content of  less than 1 vol. %, preferably even  less than 0.1 vol. %, can be obtained.
The invention provides a process for producing vacuole-free polymer granules wherein, after having been extruded, the polymer is cooled at a cooling rate of  less than 50xc2x0 C./sec, preferably  less than 20xc2x0 C., to a temperature which is 1xc2x0 C. to 20xc2x0 C., preferably 5xc2x0 C. to 10xc2x0 C., above the glass transition temperature Tg of the polymer, and then cut. In a preferred embodiment of the invention, during the cutting process the granular material is cooled to a temperature of xe2x89xa6100xc2x0 C., preferably xe2x89xa690xc2x0 C. This can be effected, for example, by spraying with water which is at a temperature of xe2x89xa630xc2x0 C.
The temperature difference between polymer and cooling medium is preferably less than 370xc2x0 C. In a preferred embodiment, the polymer is cooled with water which is at a temperature of 40xc2x0 C. to 80xc2x0 C., preferably 50xc2x0 C. to 60xc2x0 C. Particularly preferably, after having been extruded, the polymer is passed through a water bath at a temperature of 40xc2x0 C. to 80xc2x0 C., preferably 50xc2x0 C. to 60xc2x0 C. The residence time of the polymer in the water bath should preferably be 3 to 10 seconds. In an alternative embodiment, the strand of polymer is sprayed with water after the extrusion process, the water being at a temperature of 40xc2x0 C. to 80xc2x0 C., preferably 50xc2x0 C. to 60xc2x0 C.
The process according to the invention is suitable for the production of vacuole-free polymer granules of any kind. It is particularly suitable, however, for producing granules of thermoplastic polymers. For the purpose of the invention, thermoplastic polymers include all plastics which become flowable under the effects of pressure and temperature. Examples which may be given here are polystyrene, polyphenylene, polyurethane, polyamide, polyester, polyacrylate, polymethacrylate, SAN and its copolymers. The process is most particularly suitable for producing vacuole-free polycarbonate granules.
For the purpose of the present invention, polycarbonates may be either homopoly-carbonates or copolycarbonates. The polycarbonates may be linear or branched in the known manner. Up to 80 mol. %, preferably from 20 mol. % up to 50 mol. %, of the carbonate groups in the suitable polycarbonates can be replaced by aromatic dicarboxylic ester groups. Such polycarbonates, which contain both acidic groups of carbonic acid and acidic groups of aromatic dicarboxylic acids incorporated into the molecular chain, are, accurately described, aromatic polyester carbonates. They are included under the general heading of thermoplastic, aromatic polycarbonates.
Detailed information about the production of polycarbonates has been set down in hundreds of patent specifications over approximately the last 40 years. By way of example, reference is made here only to: Schnell, xe2x80x9cChemistry and Physics of Polycarbonatesxe2x80x9d, Polymer Reviews, Volume 9, Interscience Publishers, New York, London, Sydney 1964, to D.C. PREVORSEK, B.T. DEBONA and Y. KESTEN, Corporate Research Center, Allied Chemical Corporation, Morristown, N.J. 07960, xe2x80x9cSynthesis of Poly(estercarbonate) Copolymersxe2x80x9d in Journal of Polymer Science, Polymer Chemistry Edition, Vol. 19, 75-90 (1980), to D. Freitag, U. Grigo, P. R. Mxc3xcller, N. Nouvertne"", BAYER AG, xe2x80x9cPolycarbonatesxe2x80x9d in Encyclopedia of Polymer Science and Engineering, Volume 11, Second Edition, 1988, pages 648-718 and finally, to Dres. U. Grigo, K. Kircher and P. R. Mxc3xcller xe2x80x9cPolycarbonatesxe2x80x9d in Becker/Braun, Kunststoff-Handbuch, Volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester, Carl-Hanser Verlag, Munich, Vienna, 1992, pages 117-299.
Preferred thermoplastic polycarbonates have average molecular weights Mv (determined by measurement of the relative viscosity at 25xc2x0 C. in CH2Cl2 and at a concentration of 0.5 g per 100 ml CH2Cl2) of 12,000 to 400,000, preferably of 18,000 to 80,000 and in particular of 22,000 to 60,000.